Brazed aluminum alloy products, as represented by automotive heat exchangers, for obtaining well-brazed state, have required removal of oxide layers formed on their surfaces after extrusion or rolling process.
Examples of the process of removing the oxide layer include a process of removing the oxide layer by applying a chloride-based flux or a fluoride-based flux. Nowadays, a nitrogen-atmospheric (NB) brazing process using a fluoride-based flux, which does not require a washing step after brazing but leads to a lower apparatus cost, has become a mainstream in the art.
A usual process of applying the fluoride-based flux has been to apply a suspension of flux on a desired portion by means of spraying or the like, just before the brazing process after assembling members to be brazed. However, in this process, the adhesion of flux after applying is poor and thus the flux is flown by hot air that circulated in a furnace. Therefore, there is a risk of causing a problem in that the desired objective effect would not be exerted. Further, there are other problems in that floating of un-adhered flux causes at the time of applying the flux worsened working conditions and that the step of applying the flux results in an increase in production cost. Besides, since an automotive heat exchanger has a detailed complicated configuration, uniform coating of flux is very difficult. Therefore, there is a problem of inadequate brazing at a portion having an insufficient amount of flux applied, or there is a problem of spoiling the beauty of appearance with the residual flux which could not participate in a reaction at a portion having an excess amount of flux.
General methods of applying flux include a dipping method, a spray atomizing method (spraying method), a brush coating method, and a roll transfer method using a roll coater (JP-A-7-303858 (“JP-A” means unexamined published Japanese patent application)). Among them, with respect to the dipping method, it is very difficult to cope with the control of the thickness of a coated layer, especially the control of a thin film in micron order. If the precipitation property of the solid component in a coating is high, there is a problem in that a stable coated layer cannot be formed and the method is unsuitable to the formation of a coated layer at high speed (JP-A-10-94871). To that end, considering the productivity, the use of a spray method (JP-A-09-85483), a brush-coating method, or a roll transfer method as a coating method is being under study. However, the spray atomizing method may be adaptable for low-viscous coating but not suitable to high-viscous coating. Besides, there is a disadvantage in that problems such as variation in amount of the coating applied or the like tend to occur owing to a change in atomizing pattern caused by clogging a nozzle portion or generating hardened coating. In consideration of such a fact, in particular, the employment of a roll transfer method has been studied.
The roll transfer method selects a process of supplying a coating depending on the characteristics of the coating. That is, a bottom-up process in which a coating is swept up with a roll is generally selected when the coating contains a solute having a low sedimentation rate. Alternatively, for another kind of coating, such as a flux compound, in which the specific gravity of a solute is larger than that of a solvent (i.e., the solute in the coating has a high sedimentation rate), a top-feed method with which the sedimentation of the solute is prevented by supplying and circulating the coating between rolls is generally selected (JP-A-5-96230).
For the process of supplying a coating in the top-feed method, the supplying position is restricted by the number and arrangement of rolls. Further, the supplying position restricts the rotation direction of each roll. The uniformity of the coated layer after application of the coating is greatly influenced by the characteristics of the coating and the rotation direction of the roll. Therefore, those factors are important especially for designing a coating apparatus.
In recent years, attempts have been conducted to provide an aluminum alloy heat exchanger member with various functions. In one example, for the purpose of lowering the cost by bearing a fin material shaped by corrugating an aluminum alloy brazing sheet material, there is proposed a method of applying a coating on the surface of an aluminum alloy flat multi-cavity tube to be combined with the fin. In this case, the coating is prepared by mixing a filler alloy powder, as typified by an Al—Si alloy, and a flux powder, as typified by K3AlF6, in an organic binder (JP-A-11-239867). In another example, for obtaining an effect of improving corrosion resistance accompanying reductions in mass and thickness of an automotive heat exchanger, there is proposed a method of applying a coating, which is prepared by mixing into an organic binder a metal powder, as typified by Zn, having a sacrificial anode effect, or a flux compound, as typified by KZnF3, provided as an alloy having such a sacrificial anode effect (JP-A-05-96230). In the later case, for example, in a flat multi-cavity tube (202), as illustrated in a cross sectional view in FIG. 29, even though application of the coating of a filler material or a sacrificial anode effect alloy to flat portion (202F) in contact with the fin material (e.g., 16 in FIG. 10) is clearly described, there is no clear description particularly with respect to the technology of applying the coating on curved portion (202R) not in contact with the fin material. However, the presence or absence of coated layers on the flat portion and the curved portion causes an electrochemically imbalanced condition. In particular, in the case of the application of the coating of a sacrificial anode effect alloy, as the sacrificial anode effect alloy does not exist on the curved portion, an electrical potential became electropositive as compared with that of the flat portion, and thus corrosion such as pitting corrosion occurred at an early stage, resulting in serious problems in the design of corrosion protection. On the other hand, in both of those two cases, as the coating contains dispersions of the metal powder and the flux compound each having a specific gravity larger than that of the organic binder, important subject matters to be solved are securing the stability of the coating at the time of coating with the roll transfer method and securing the uniformity of the coated layer.
For instance, an aluminum alloy member for an automotive heat exchanger, particularly a product like a flat multi-cavity tube is generally rolled in the shape of a coil in view of productivity. Thus, when coating such as a flux compound is applied on the surface of such a coil-shaped product, an important quality item is the dryness of the coating after application. If the dryness is insufficient, troubles in brazing and corrosion resistance may occur owing to a decrease in adhesion of the coating and peal of the coating at the time of assembly by brazing. A process of drying a coating applied by means of a roll transfer method usually used in the art is a hot air system or a far-infrared heater system. Since the hot air system brows hot air directly or indirectly to the surface of a product after application of the coating, vibration of the product may occur. As the vibration spreads in a roll transfer part, the mass of the coating adhered tends to vary, which is a defect. Further, if the speed of the product is high, there is a problem in that the length of a drying furnace should be extended because of insufficient drying capacity. The far-infrared heater system does not generate the vibration unlike the hot air system, but, similar to the hot air system, causes an insufficient drying capacity at high-speed coating.
On the other hand, a brazing joint of an aluminum heat exchanger for an automobile, such as a condenser, is generally carried out such that an aluminum-extruded flat multi-cavity tube and a brazing sheet prepared by cladding an Al—Si-based filler material on an aluminum material are assembled into a predetermined joint configuration and then heated at a brazing temperature.
In this case, for attaining a sufficient joint, it is necessary to destroy and remove a hard aluminum oxide on the surfaces of the filler material of the extruded flat multi-cavity tube and aluminum brazing sheet. In general, flux is suspended in water or an alcohol in advance, the resultant suspension is applied by spraying on the brazing joint surface, and then a solvent is vaporized, followed by brazing.
In recent years, in view of dispensing with the flux coating work just before brazing, methods (pre-coating) of applying, in advance, a flux composition or a mixture composition of flux with a filler material on the surface of a material to be brazed, and brazing compositions for pre-coating have been proposed in JP-A-3-35870, JP-A-6-285681, JP-A-6-504485, Japanese Patent No. 2681380, Japanese Patent No. 2681389, and the like.
Further, these pre-coating methods include coating methods such as a spray method, a dipping method, and a roll transfer method. In these methods, compositions having a certain composition ratio tend to find difficulty in high speed coating because of precipitation of metal powder. For uniformly pre-coating a flux composition or a mixture composition of a flux and a filler material successively at high speed with good transferring property and adhesion by the roll transfer method, JP-A-11-239867 proposes: a brazing composition in which the type of a synthetic resin in the composition and/or a mass ratio between the flux or filler material and the synthetic resin in the composition are specified; and a production method in which the properties of an organic solvent and the viscosity of the coating are specified.
However, when the brazing composition having such a coating composition and coating property is applied by a usual roll transfer method, it cannot be uniformly pre-coated at high speed in a continuous fashion with good transferring property and adhesion. Thus, a resin-coated aluminum flat multi-cavity tube that is coated with a composition having a certain composition ratio in a stable manner cannot be obtained.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.
Herein, the identical reference numerals in the drawings refer to the same components or members.